Bugfix for extension methods: Treat System.Private.CoreLib as a system lib for extension method search

docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

@@ -1,5 +1,5 @@
 ### Fixed
-
+* Fix extension methods support for non-reference system assemblies ([PR #17799](https://github.com/dotnet/fsharp/pull/17799))
 * Ensure `frameworkTcImportsCache` mutations are threadsafe. ([PR #17795](https://github.com/dotnet/fsharp/pull/17795))
 
 ### Added

src/Compiler/AbstractIL/ilread.fs

@@ -2144,11 +2144,6 @@ and typeDefReader ctxtH : ILTypeDefStored =
             else
                 let mutable attrIdx = attrsStartIdx
 
-                let looksLikeSystemAssembly =
-                    ctxt.fileName.EndsWith("System.Runtime.dll")
-                    || ctxt.fileName.EndsWith("mscorlib.dll")
-                    || ctxt.fileName.EndsWith("netstandard.dll")
-
                 while attrIdx <= attrsEndIdx && not containsExtensionMethods do
                     let mutable addr = ctxt.rowAddr TableNames.CustomAttribute attrIdx
                     // skip parentIndex to read typeIndex
@@ -2159,12 +2154,9 @@ and typeDefReader ctxtH : ILTypeDefStored =
                     let name =
                         if attrTypeIndex.tag = cat_MethodDef then
                             // the ExtensionAttribute constructor can be cat_MethodDef if the metadata is read from the assembly
-                            // in which the corresponding attribute is defined -- from the system library
-                            if not looksLikeSystemAssembly then
-                                ""
-                            else
-                                let _, (_, nameIdx, namespaceIdx, _, _, _) = seekMethodDefParent ctxt attrCtorIdx
-                                readBlobHeapAsTypeName ctxt (nameIdx, namespaceIdx)
+                            // in which the corresponding attribute is defined
+                            let _, (_, nameIdx, namespaceIdx, _, _, _) = seekMethodDefParent ctxt attrCtorIdx
+                            readBlobHeapAsTypeName ctxt (nameIdx, namespaceIdx)
                         else
                             let mutable addr = ctxt.rowAddr TableNames.MemberRef attrCtorIdx
                             let mrpTag = seekReadMemberRefParentIdx ctxt mdv &addr

tests/FSharp.Compiler.Private.Scripting.UnitTests/FSharpScriptTests.fs

@@ -352,6 +352,22 @@ tInput.Length
         let value = opt.Value
         Assert.Equal(4L, downcast value.ReflectionValue)
 
+    [<FSharp.Test.FactForNETCOREAPP>] // usessdkrefs is not a valid option for desktop compiler
+    member _.``ML - use assembly with ref dependencies and without refing SMemory``() =
+        let code = """
+#r "nuget:Microsoft.ML.OnnxTransformer,1.4.0"
+
+open System
+open System.Numerics.Tensors
+let inputValues = [| 12.0; 10.0; 17.0; 5.0 |]
+let tInput = new DenseTensor<float>(inputValues.AsMemory(), new ReadOnlySpan<int>([|4|]))
+tInput.Length
+"""
+        use script = new FSharpScript(additionalArgs=[| "/usesdkrefs-" |])
+        let opt = script.Eval(code)  |> getValue
+        let value = opt.Value
+        Assert.Equal(4L, downcast value.ReflectionValue)
+
     [<Fact>]
     member _.``System.Device.Gpio - Ensure we reference the runtime version of the assembly``() =
         let code = """